(LifeScript Health News) People in their early 50s and older are more resistant to the swine flu virus, a new study says.

Blood tests of people born before 1957 show they have antibodies that are attacking the virus, according to the Centers for Disease Control and Prevention (CDC). One theory for the resistance is that people born before 1957 were exposed to flu “ancestors” of the current H1N1 swine virus. Until 1957, most seasonal flu strains were descendents of the 1918 Spanish flu, which was also H1N1. Current seasonal flu shots target later viruses, including a different H1N1 strain, H3N2 and a B strain. There are no flu shots to protect against the current swine flu virus.

Still, the study results don’t indicate that people over age 52 are immune to swine flu. The CDC recommends that older people take the same precautions as younger folks.